2,015 research outputs found
The Nature of [Ar III] Bright Knots in the Crab Nebula
The kinematic and morphological properties of a string of [Ar III] bright
knots in the Crab Nebula are examined using 1994 - 1999 HST WFPC-2 images of
the remnant. We find that five southern [Ar III] bright knots exhibit ordinary
radial motions away from the nebula's center of expansion with magnitudes
consistent with their projected radial displacements. These results do not
support the suggestion by MacAlpine et al.(1994) that these knots might be
moving rapidly away from the Crab pulsar due to a collimated wind. The HST
images also do not show that the [Ar III] knots have unusual morphologies
relative to other features in the remnant. Our proper motion results, when
combined with radial velocity estimates, suggest these knots have relatively
low space velocities implying relatively interior remnant locations thus
placing them closer to the ionizing radiation from the Crab's synchrotron
nebula. This might lead to higher knot gas temperatures thereby explaining the
knots' unusual line emission strengths as MacAlpine et al.(1994) suspected.Comment: 11 pages including three figures. Submitted to the Astronomical
Journa
Optical properties of SiC nanotubes: A systematic study
The band structure and optical dielectric function of
single-walled zigzag
[(3,0),(4,0),(5,0),(6,0),(8,0),(9,0),(12,0),(16,0),(20,0),(24,0)], armchair
[(3,3),(4,4),(5,5),(8,8),(12,12),(15,15)], and chiral
[(4,2),(6,2),(8,4),(10,4)] SiC-NTs as well as the single honeycomb SiC sheet
have been calculated within DFT with the LDA. It is found that all the SiC
nanotubes are semiconductors, except the ultrasmall (3,0) and (4,0) zigzag
tubes which are metallic. Furthermore, the band gap of the zigzag SiC-NTs which
is direct, may be reduced from that of the SiC sheet to zero by reducing the
diameter (), though the band gap for all the SiC nanotubes with a diameter
larger than ~20 \AA is almost independent of diameter. For the electric
field parallel to the tube axis (), the for
all the SiC-NTs with a moderate diameter (say, 8 \AA) in the
low-energy region (0~6 eV) consists of a single distinct peak at ~3 eV.
However, for the small diameter SiC nanotubes such as the (4,2),(4,4) SiC-NTs,
the spectrum does deviate markedly from this general behavior. In
the high-energy region (from 6 eV upwards), the for all the
SiC-NTs exhibit a broad peak centered at ~7 eV. For the electric field
perpendicular to the tube axis (), the spectrum of
all the SiC-NTs except the (4,4), (3,0) and (4,0) nanotubes, in the low energy
region also consists of a pronounced peak at around 3 eV whilst in the
high-energy region is roughly made up of a broad hump starting from 6 eV. The
magnitude of the peaks is in general about half of the magnitude of the
corresponding ones for
Tunability of the optical absorption in small silver cluster-polymer hybrid systems
We have calculated the absorption characteristics of different hybrid systems
consisting of Ag, Ag2 or Ag3 atomic clusters and poly(methacrylic acid) (PMAA)
using the time-dependent density-functional theory. The polymer is found to
have an extensive structural-dependency on the spectral patterns of the hybrid
systems relative to the bare clusters. The absorption spectrum can be `tuned'
to the visible range for hybrid systems with an odd number of electrons per
silver cluster, whereas for hybrid systems comprising an even number of
electrons, the leading absorption edge can be shifted up to about 4.5 eV. The
results give theoretical support to the experimental observations on the
absorption in the visible range in metal cluster-polymer hybrid structures.Comment: Updated layout and minor changes in versions 2 and
Density-functional investigation of the rhombohedral to simple cubic phase transition of arsenic
We report on our investigation of the crystal structure of arsenic under
compression, focusing primarily on the pressure-induced A7 to simple cubic (sc)
phase transition. The two-atom rhombohedral unit cell is subjected to pressures
ranging from 0 GPa to 200 GPa; for each given pressure, cell lengths and
angles, as well as atomic positions, are allowed to vary until the fully
relaxed structure is obtained. We find that the nearest and next-nearest
neighbor distances give the clearest indication of the occurrence of a
structural phase transition. Calculations are performed using the local density
approximation (LDA) and the PBE and PW91 generalized gradient approximations
(GGA-PBE and GGA-PW91) for the exchange-correlation functional. The A7 to sc
transition is found to occur at 21+/-1 GPa in the LDA, at 28+/-1 GPa in the
GGA-PBE and at 29+/-1 GPa in the GGA-PW91; no volume discontinuity is observed
across the transition in any of the three cases. We use k-point grids as dense
as 66X66X66 to enable us to present reliably converged results for the A7 to sc
transition of arsenic.Comment: To be published in Physical Review B; material supplementary to this
article is available at arXiv:0810.169
Calculated phonon spectra of paramagnetic iron at the alpha-gamma phase transition
We compute lattice dynamical properties of iron at the bcc-fcc phase
transition using dynamical mean-field theory implemented with the frozen-phonon
method. Electronic correlations are found to have a strong effect on the
lattice stability of paramagnetic iron in the bcc phase. Our results for the
structural phase stability and lattice dynamical properties of iron are in good
agreement with experiment.Comment: 4 pages, 2 figure
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